What is a Plant, Its Characteristics, and Various Main Groupings

What is a plant? Several basic questions are relevant in reviewing crop farming or crop agriculture as a science and practice. These include what is: agriculture, crop, weed, agronomy and horticulture, as well as the various factors and processes involved in plant growth and development. But equally necessary is to have a wide understanding of plants and their characteristics. This is so because crop production generally means plant production.

A plant is a multicellular, autotrophic living organism capable of photosynthesis, adapted to living on land, and belongs to the kingdom Plantae. Planta or plantae is also the Latin word for plant, sprout, shoot, slip, young plant, seedling, sole and foot (Kidd 1957; Traupman 1995).

Although there are aquatic plants, the main adaptation of plants is for living on land. In other words, they are primarily terrestrial dwellers. Living in water is only a secondary adaptation (Mader 1993).

In addition to Plantae, there are at least four other kingdoms (regnum) of living organisms (Moore et al. 2003). The other kingdoms with common acceptance are: Animalia (animals), Monera (bacteria, procaryotic), Fungi (fungi), and Protoctista (algae, protozoans and slime molds). Other organisms placed under kingdom Protoctista are those which do not fit into the other kingdoms.

The uses of plants are without limit and, if comprehensively enumerated, would not fit into this page. But their primary importance is that they are vital to life. They are a basis of the food pyramid for living organisms. They are the source of energy for most heterotrophic organisms. They likewise generate oxygen which is essential to aerobic organisms including humans and which makes the atmospheric conditions favorable for life. Even when some plants are considered weeds in agriculture, they still perform these life-sustaining functions.

What is a Plant: Characteristics

There are many more ways of describing what is a plant. Here are some of their distinct
characteristics:

1. Plants
are multicellular organisms made up of eucaryotic cells. These cells also occur
in other organisms except bacteria and cyanobacteria. Eucaryotic cells have membrane-bound
organelles and a nucleus. They have chloroplasts that contain chlorophylls a
and b and carotenoids; and cell walls which are rich in cellulose.

2.
Plants are generally autotrophic, oxygen-evolving photosynthesizers. They produce their own food
(source of energy and carbon skeletons) through photosynthesis in which process
oxygen is given off. This is the most important characteristic that describes
what is a plant in relation to crop farming and in perpetuating life on Earth (click to read What is Photosynthesis).
Starch is the primary storage product of photosynthesis and their food reserve.

3. In
contrast to other organisms, plants are non-motile. However, plants are not
exactly immobile. They also exhibit some form of limited movement like the
unidirectional movement of plant parts in response to light, or phototropism, and the folding of the
leaves of Mimosa in response to
touch.

4. The
life cycle of plants consists of a distinct alternation
of generations. This means that plants have both the haploid gametophyte which produces gametes and
the diploid sporophyte. Depending on
the species, the generations can occur simultaneously or successively. Except
in the bryophytes, the dominant generation is the sporophyte.

5. Along
with the sporophyte, plants have evolved the embryo, an immature diploid sporophyte that is attached to or
surrounded by the gametophyte (Simpson 2010). Thus the term Embryophyta or
embryophytes, also called land plants
as distinguished from the former green plants which include the “green algae.”

5.
Plants, like animals, possess an internal protective mechanism by which drying
of the zygote is prevented. This is accomplished by containing the zygote in an
internal organ during development (click
to read also Functions of Fruits). All plants possess adaptations which protect the embryo from drying out, a characteristic that is absent and which differentiates them from the green alga Ulva (Mader 1993).

6.
Unlike animals, plants have no nervous system and organized muscle fibers.

In
addition, plants are classified into different grouping based on various
criteria. Some general classifications are provided below.

*According to Simpson (2010), the exact placement of the gnetophytes (Gnetophyta, Gnetales or Gnetopsida), that is, whether separate or under Coniferae (or Pinophyta) is still contested.

What is a plant: nonvascular vs. vascular plants

Nonvascular plants are those without vascular tissues responsible for the internal transport systems that have evolved in later plants. They consist of the bryophytes under the divisions Bryophyta, Hepatophyta and Anthocerotophyta. All other plants are vascular plants.

Nonvascular plants have no true roots but instead have root-like rhizoids. They absorb and transport water, minerals and organic nutrients by diffusion. A moist environment is therefore essential to their growth and development. (Practical application: The use of moss as a groundcover in bonsai culture and presentation serves dual purposes: besides the aesthetic enhancement, moss is also a good indicator of insufficient soil moisture. They easily turn brownish when soil water is limiting.)

In contrast, the vascular plants have evolved vascular tissues consisting of the xylem and phloem. The xylem transports water and minerals and the phloem transports organic nutrients throughout the plant body.

What is a plant: seedless vs. seed plants

Plants either
possess the natural ability to produce seeds or not. The seed is a plant organ and has been defined as an embryo surrounded
by nutritive tissue and enveloped by a seed coat (Simpson 2010).

The seedless plants consist of both the
nonvascular plants and vascular plants. Their mode of reproduction is by spore. The nonvascular seedless plants are the bryophytes (divisions Bryophyta,
Hepatophyta and Anthocerophyta) while the vascular seedless plants consist
of the ferns and allies (divisions Psilotophyta,
Lycopodophyta, Equisetophyta and Pteridophyta).

On the
other hand, the seed plants or
spermatophytes have evolved the
ability to produce seeds and consist of the plants informally called
gymnosperms and angiosperms.